Fix multipart inserts of attachments containing unicode chars.

Previously, couchdb would throw an error when attempting to do multipart
inserts of attachments containing unicode characters. This was caused by
the Content-Length header being incorrectly set to the number of
characters of the attachment (via `data.length`) rather than the number
of bytes (using `Buffer.byteLength(data)`).
Merge pull request #225 from tobie/fix-unicode-attachments

Fix multipart inserts of attachments containing unicode chars.